In the process of doubling the frequency of high-power laser radiation in metamaterials, the self- and cross-phase modulations associated with the cubic nonlinearity of the medium are studied in the constant-intensity approximation. It is assumed that the pump wave propagates in a region with negative refraction. It is shown that in the case of a nonlinear-optical process of frequency doubling of an intense light wave in a metamaterial, it is important to take into account the self-action and cross-interaction effects that directly affect the optimal phase relationship between the interacting waves. It is discovered that, as a result of counter interaction of waves, the period of spatial beats changes. According to the analytic expressions obtained in the work, the choice of optimal parameters of the problem makes it possible to realize the regime of effective frequency doubling. By varying the intensity of the pump, it is possible to control and manipulate the intensity of the output coherent radiation from the minimum to the maximum value. A numerical evaluation of the efficiency of the doubling process in a metamaterial is given, and a comparison is made with similar results in LiNbO3 and KDP crystals. It is demonstrated that the effects of self- and cross-modulation in media with negative refraction can be used as a powerful tool for developing effective frequency doublers in such materials.